In order to produce microelectronic components or printed circuit boards, mass production patterning methods are employed in which the structures that are to be imaged, for example on a silicon wafer for the fabrication of chips, are imaged through the use of masks, preferably made of metal-coated quartz glass. The wafer is patterned by coating its surface with a photoresist and partially exposing it, through the use of the masks. This form of patterning, known as photolithography, is a very mature technology and is therefore widespread in mass production.
When producing smaller numbers of chips, for example for test series, or when fabricating gate array circuits, the comparatively high outlay for the production of the multiplicity of masks required for fabricating a circuit has a disadvantageous effect from an economic standpoint. The fabrication of the masks can be time-consuming and costly. Added to this is the fact that new masks continually have to be designed and fabricated because of the short innovation cycles in microelectronics and the hence frequently changing production profiles of semiconductor manufacturers. Therefore, for some time there has been a transition partly toward maskless production of the corresponding structures. A technology that can be used for this is electron beam writing, for example. In this case, the radiation source, an electron beam, is driven directly in the manner required for the structure to be produced. However, this technology requires a high outlay on apparatus and, for this reason, is likewise too expensive for the production of smaller series.
WO 93/09472 specifies a technical solution in which the maskless patterning, as known from the technologies which work with masks, is effected using light. The aforementioned document describes a method and an apparatus in which the light beam emerging from a light source is modulated by surface light modulators. The light beam is guided onto an imaging element and modulated by said imaging element with regard to its areal propagation and the radiation modulated in this way is fed to the layer to be patterned.
In accordance with the solution presented in the document, the imaging element is a special mirror chip which has, below its surface, a multiplicity of microelectrodes that are arranged in a matrix and can be driven individually. The layer located beneath the chip surface of the mirror chip is deformed partially depending on the driving of said electrodes. At the deformed regions, the light emitted by the light source for patterning is diffracted, so that the light is only reflected by the non-driven regions directly and without diffraction.
By means of a suitable optical arrangement with a semitransparent mirror and a diaphragm, all of the diffracted light components are then filtered out and only the directly reflected light is guided onto the light-sensitive layer to be patterned. As a result, a negative image of the driving state at the imaging element is produced on the light-sensitive layer. In other words, a structure corresponding to the non-driven electrodes (pixels) of the mirror chip is imaged there.
In the practical implementation of the method, the light-sensitive layer is patterned in a plurality of exposure operations. As a result, statistical influences which result for example from the energy statistics of the laser that is preferably used for the exposure, or from other statistical influences, such as undesirable alterations to the present focus position, are eliminated to the greatest possible extent. In this case, the total exposure dose (nominal exposure dose) required for producing a structural element or groups of structural elements or parts thereof is divided between the individual exposure operations.
According to a known procedure, the process for this purpose involves an exposure with 50% of the exposure dose that is required in total being effected during the first exposure operation and the exposure dose being halved in each subsequent exposure operation. Since the exposure dose that is required in total must result altogether after all of the exposure operations have been carried out, this means that it is necessary to perform the exposure operation with the lowest exposure dose (intensity) twice.
The variation of the exposure doses in the individual exposure operations is necessary for obtaining different gray-scale values of the imaging in order to achieve a predetermined addressing grid. However, each additionally required exposure operation has a disadvantageous effect with regard to the writing speed at which the structure is transferred to the light-sensitive layer. Moreover, the occurrence-which cannot be precluded—of pixel defects at the imaging element impair the image quality of the structural elements imaged on the light-sensitive layer.
Although possible solutions are already offered in respect of this in WO 93/09472, these are still not satisfactory in part with regard to the result or the outlay. Further inaccuracies or impairments of the image quality result when image fields are placed together in order to produce more extensive structures inter alia due to the finite numerical aperture of the optical arrangement.
Object
Therefore, it is an object of the invention to specify a method by which any of: the abovementioned disadvantages are avoided. In particular, the intention is to demonstrate possibilities for increasing the writing speed and/or for improving the image quality.